Optical coherence tomography ("OCT") apparatus are well known in the prior art. Such OCT apparatus can measure with micrometer resolution, and a typical example of such a prior art OCT apparatus includes a low coherence light source and a modified Michelson interferometer. Various embodiments of such prior art OCT apparatus have been developed for use in analyzing optical fibers and for use in medical applications to investigate biological tissue such as, for example, tissue in a human eye.
A typical OCT apparatus fabricated in accordance with the prior art splits radiation output from the low coherence light source into a reference beam and a probe beam. The reference beam is typically directed to a reference path which includes a device that varies the optical pathlength of the reference beam, the device typically being a moving reflector. The probe beam, on the other hand, is typically directed to a sample path which causes the probe beam to impinge upon a sample to be investigated. Radiation backscattered from various scattering centers in the sample is collected in the sample path. Next, the backscattered radiation output from the sample path is combined with radiation from the reference beam that is output from the reference path. The combined radiation is directed to impinge upon a detector.
Due to the low coherence of the radiation output from the low coherence light source of the typical prior art OCT apparatus, the detector only yields signals of interest whenever the optical pathlengths of the reference radiation output from the reference path and the backscattered radiation output from the sample path are substantially the same; within the coherence length of the radiation output from the low coherence light source.
The above-described prior art OCT apparatus has been used in medical applications with the objective of providing three dimensional ("3D") images of in vivo biological tissues with micrometer precision. However, problems arise with the use of the prior art OCT apparatus in such medical applications. The most important problem arises as a result of the relatively slow movement of a movable mirror that has typically been used to vary the optical pathlength of the reference path; the problem relates to uncontrollable motion of human tissue and to system noise. This problem makes it difficult, if not impossible, to achieve the objective of providing three dimensional images of in vivo biological tissues, especially when that objective includes mapping dynamic biological tissue such as that found in the human eye. For example, an article entitled "Optical Coherence Tomography of the Human Retina" by M. R. Hee, J. A. Izatt, E. A. Swanson, D. Huang, J. S. Schuman, C. P. Lin, C. A. Puliafito, and J. G. Fujimoto, in Arch. Ophthalmol., Vol. 113, March 1995, pp. 325-329 discloses that "The 200 (horizontal).times.250 (vertical) pixel image was acquired in 5 seconds and corresponds to a 5.6-mm cross section along the papillomacular axis of the retina." Thus, it would require over 16 minutes to use the disclosed OCT apparatus to provide a three dimensional map of the desired object, for example, a 200.times.200.times.250 pixel image. It should be readily apparent that this is impractical because it is almost impossible for a human subject to hold his/her eye motionless for such a long time. As a result, prior art OCT apparatus that only use a movable mirror to vary the optical pathlength of a reference path cannot solve the above-identified problem of providing 3D images of in vivo biological tissues.
One suggestion in the prior art for an alternative to using a moving reflector is to modulate radiation emitted by the low coherence light source with a linear frequency modulation (FM) chirp. Although this suggestion would remove the need for a movable reflector, it creates still other problems. For example, in order for the OCT apparatus to provide micrometer resolution, the FM chirp needs to be more than ten percent (10%) of the central source frequency. However, such a wideband FM chirp cannot be provided with present technology.
A further problem that occurs with the prior art OCT apparatus occurs as a result of the fact that the resolution depends on the bandwidth of the radiation output by the low coherence light source. For examining biological tissue, a preferred light source should output radiation that is not absorbed by common constituents of biological tissue such as, for example, water and melanin. This is to enable the radiation to penetrate deeply into the sample tissue and to enable the OCT apparatus to provide images of tissues having a small backscattering cross section. The further problem is that only certain light sources output radiation in suitable frequency bands. Additionally, a still further problem occurs in that, even if the low coherent light source outputs radiation in the desired frequency band, the output radiation may not have enough power to enable the OCT apparatus to take advantage of the ability of the output radiation to penetrate deeply into the sample.
In light of the above, there is a need in the art for a high speed, high precision inspection method and apparatus that overcomes the above-identified problems.